THE INDIA RUBBER WORLD 



[May I, 1916. 



The parts in the ignition system that may be made wholly 

 or in part from hard rubber are distributor blocks, collector 

 rings and brush holders for magnetos, terminal connections and 

 miscellaneous parts for magnetos, coils and switches. These 

 parts are molded, as a rule, and the comments already made 

 concerning the superiority of hard-metal over soft-metal molds 

 apply. 



It is a comparatively easy matter to make these parts from 

 material that will withstand 10,000 volts per 1-32 inch of thick- 

 ness and the insulation problem is therefore not difficult. To 

 furnish material that will withstand a jump spark is another 

 problem, for the heat of the spark tends to carbonize the rubber. 

 The Germans are able to furnish a rubber compound that in 

 this respect seems to be superior to any it has been possible to 

 produce in this country. 



llARD-RUnnKR P.ARTS FOR BATrERY EQUIPMENT. 



Parts for batteries include hard rubber jars, covers, vents, 

 bushings and separators. The rtiore complicated hard rubber 

 covers must be molded. The less complicated ones may be 

 molded or cut from hard rubber sheet. The greater accuracy 

 of dimensions that can be maintained with hard-metal molds 

 is of greater advantage here than in connection wuth the parts 

 previously mentioned. Most of the other parts must be finished 

 after they are molded in order to give them the required ac- 

 curacy of dimensions or character of surface. It is customary 

 to grind the edges of molded covers fairly accurately to dimen- 

 sions, but to leave the other surfaces practically as they come 

 from the molds. To finish the other surfaces would greatly in- 

 crease the cost of manufacture. In other words, molded covers 

 must be molded fairly accurately, and this can be done more 

 satisfactorily with hard-metal than with soft-metal molds. 



Flat covers are usually cut from sheet, but when large in size. 

 and the quantity required is large enough to warrant the cost 

 of equipment, it is often advantageous to mold them. When the 

 same compound is used, a stronger cover can be made by mold- 

 ing than by cutting it from sheet, and this is a factor in de- 

 termining the better method of manufacture in order to produce 

 a cover of a given quality. 



The cost per cover for operating the molds is practically the 

 same, regardless of the size of the cover. Consequently it often 

 happens that of a certain type of flat cover it is most economical 

 to cut the small ones from sheet and to mold the larger ones. 

 Hard rubber sheet from which covers are made is vulcanized 

 between layers of tin in hot water. These sheets are about 

 20 X 48 inches, the exact size, of course, being detertnined so that 

 there may be no excessive waste at the edges. 



Battery manufacturers are probably more inclined to econo- 

 mize beyond the point of safety in the cost of covers than of 

 other parts of a battery. The hard rubber cover is so small 

 that the addition of a cent or two in the cost of material makes 

 a considerable difference in its strength, and often a cent or two 

 may make a very great difference in the breakage in service. 



METHODS OF MAKING VENTS. 



There are two types of hard rubber vents in common use, 

 plain molded and blown. The "plain molded" is made in the 

 same manner as the other molded parts previously mentioned. 

 The "blown" vents are made hollow by the expansion of water 

 or other liquids on the inside, this expansion being caused by 

 the application of heat during vulcanization. It is more ex- 

 pensive to mold goods when they are hollow. More expensive 

 raterial is required. Of course, in some designs it is essential 

 that vents be made hollow, but wherever it is possible in order 

 to reduce costs, vents should be so designed that all holes and 

 irregularities can be made by the means of molds. 



Such items as hard rubber terminal bushings and vent exten- 

 sions are usually cut from hard rubber tubing, although here 

 again if the design is complicated and the quantity involved is 

 large it is more economical to mold them. 



Hard rubber rods and tubes are made in much the same man- 

 ner as soft rubber tubing that does not have cloth insertion. It 

 is squirted through a die while hot, the die forming it int.. the 

 desired shape. The material is then buried in soapstone. pr.jper 

 care being exercised to insure its being kept straight, tlie hard 

 rubber rod by the way it is packed, and hard rubber tubing by 

 mandrels on the inside. The standard length for hard rubber 

 rod and tubing is 30 inches. As it comes from the process of 

 vulcanization it is fairly rough and not altogether uniform_ in 

 dimensions. With a grinding machine of rather simple design, 

 hard rubber rod and tubing can be ground accurately to out- 

 side dimensions and polished at no great expense. The actual 

 cost of manufacture, per pound, of ground and polished hard 

 rubber tubing varies according to the weight per foot, that is. 

 according to the diameter and thickness of wall. This is due 

 to the fact that labor cost for grinding and polishing depends 



upon the length and number of pieces rather than upon the 

 weight. The hole in hard rubber tubing can be kept accurately 

 centered only by turning it on a mandrel in the lathe with a 

 special grinding machine, which, of course, increases the cost 

 considerably. 



TYPES OF BATTERY SEPAR.\TORS. 



In lighting and starting batteries, hard rubber separators have 

 been almost altogether supplaiited by treated-wood separators. 

 Battery manufacturers tell us that with a battery of a given 

 size tliev can get a higher discharge rate with the wood than 

 with the rubber separators, and that if they used the latter they 

 would have to increase the sizes of their batteries. 



Separators manufactured in Europe are usually made from 

 corrugated perforated hard rubber sheet, or from perforated 

 sheet with ribs molded integrally upon it. The corrugated sep- 

 arator is not altogether desirable because it will flatten out in 

 service. The separators with the ribs molded integrally with 

 the sheet have been too expensive for most American battery 

 manufacturers. At the present time, however, hard rubber 

 manufacturers are aiming to reduce their costs of manufacture 

 for that type to a point where battery manufacturers can afford 

 to use them. 



Ribbed hard rubber separators as they are made in this coun- 

 try today usually consist of hard rubber sheet partly cured and 

 perforated, and ribs vulcanized on the sheet afterwards. This 

 type of separator is more durable than are the treated wood 

 separators, while the separator with ribs molded integrally is 

 still more durable. 



Hard rubber jars as used in this country are formed over 

 mandrels and vulcanized while still on the mandrels. The jars 

 are not put into molds, but are simply wrapped with a layer of 

 tin on the outside. The rubber manufacturer is frequently asked 

 if this wrapping could not be dispensed with, thereby decreasing 

 the cost of manufacture. The tin, however, is not a great item 

 of expense, because it can be used over and over again. It 

 serves to hold the jar in shape during the vulcanizing process, 

 and offers a convenient means for branding, for it is only nec- 

 essary to make the impression with a die. and that is held by 

 the tin in the compound while the jar is being vulcanized. With- 

 out the tin the jar would have a rough instead of a fairly smooth 

 and well polished surface, dimensions could not be so well con- 

 trolled, and the expense of straightening the jars after vulcani- 

 zation would just about offset the cost of using the tin. 



THICKNESS OF JAR WALLS. 



The thickness of wall required for a hard rubber jar in auto- 

 mobile work in order to give the proper service, depends upon 

 the quality of rubber used. Walls % inch thick are now fairly 

 standard for starting and lighting work. Walls 3/32 inch thick 

 are sufficient, provided the rubber compound is good enough. 

 In fact, 1/16 inch thickness of wall, if only the finest grade of 

 materials were used, might be satisfactory, but so thin a wall 

 is not recommended because such thin material is difficult to 

 handle satisfactorily in the manufacture of a jar, and the per- 

 centage of defective jars is too great. 



There is a tendency on tlie part of some battery manufacturers 

 to use walls 3/32 inch thick in the smaller batteries for the 

 smaller and cheaper cars. While the weight per cell of the ele- 

 ments is. of course, smaller for a smaller battery, these batteries 

 are usually used on small cars where the spring suspension re- 

 lieves the ordinary strains upon a hard rubber jar to a less ex- 

 tent. To give the same service, the hard rubber jar for the small' 

 battery should have as thick a wall as in a large battery. 



JAR BREAKAGE CAUSED BY ASSEMBLING. 



.•\ great deal of the breakage of hard rubber jars in service 

 has been dqe not to the quality of the jar. but to the way it was 

 assembled. If the wooden boxes are not square and fairly rigid, 

 the bottom of the jar is frequently only partly supported. This 

 is also true where the sealing compound is not distributed uni- 

 formly over the bottom of the battery box. In such cases a jar 

 even of good quality will break either at the bottom or in the 

 wall at the point where the top of the bottom rib joins the 

 wall. 



PHYSIC. \L AND ELECTRICAL PROPERTIES. 



Many battery and rubber manufacturers have been inclined to 

 talk loosely about tensile strength, elongation and dielectric 

 strength. Statements in this connection mean practically noth- 

 ing unless all the test conditions are known. To say that a hard 

 rubber jar has a tensile strength of 3,600 pounds or more, does 

 not necessarily mean that it is a serviceable jar. The tensile 

 strength of a jar w-ill vary according to the temperature of the 

 test piece at the time the test is made. The usual temperature- 

 is 70 degrees F. .\t a lower temperature a piece of hard rub- 



